Crystal Plasticity Finite Element Models for Predicting Deformation and Twinning in Polycrystalline Magnesium Alloys

Magnesium alloys;Deformation twinning;Polycrystalline Microstructure;Crystal plasticity;Finite element anaysis;Mechanical Engineering

Cheng, JiahaoGhosh, Somnath ;

Johns Hopkins University

关键词: Magnesium alloys;    Deformation twinning;    Polycrystalline Microstructure;    Crystal plasticity;    Finite element anaysis;    Mechanical Engineering;   

Others  :  https://jscholarship.library.jhu.edu/bitstream/handle/1774.2/39728/CHENG-DISSERTATION-2016.pdf?sequence=1&isAllowed=y

瑞士|英语

来源: JOHNS HOPKINS DSpace Repository

【 摘 要 】

Magnesium alloys exhibit complex deformation related mechanical behavior, viz. plastic anisotropy, tension-compression asymmetry and premature failure. Their origins are in the underlying heterogeneous deformation due to dislocation slip and deformation twinning on different crystallographic systems. In the polycrystalline microstructure, the twin formations lead to strain localization and initiation of short cracks. Reliable prediction of mechanical response and material failure is predicated upon the ability of computational models of polycrystalline microstructures to accurately simulate the heterogeneous twin formations and deformation. In this dissertation, a novel crystal plasticity finite element (CPFE) model is developed to predict the micro-mechanism and microstructure induced material failure in Mg alloys. The model accounts for dislocation slips, explicit micro-twin nucleation-propagation, geometrically necessary dislocation (GND) accumulation and stress/strain localization in the evolving microstructure. The micro-twin nucleation is modeled based on energy-partitioning following the dislocation dissociation process. The micro-twin propagation and associated localized plastic flow are described with micro-mechanism based crystal plasticity constitutive relations. The model predicts the twin-evolution-induced material responses at multi-scales, including at macroscopic scale the tension-compression asymmetry and the dramatic change of hardening rates due to twin activities, as well as at microscopic scale the strain localization and stress redistribution in the twined microstructure which is potentially responsible for short-crack initiation. The proposed model is implemented with a new developed stabilized tetrahedral element, which avoids inaccuracy of micro-twin nucleation prediction due to element-locking. The simulation time steps are constrained by the high propagation rate of micro-twins and are several orders smaller than that of regular CPFE simulations. This increases the computation cost enormously and impedes high-fidelityCPFE simulation. A multi-time-scale subcycling algorithm for temporal integration of CPFE-twin model is innovated to improve the simulation efficiency. The algorithm divides the FE domain into sub-domains and computes local deformationin separate rates, followed by the coupling of residual forces to satisfy the global equilibrium. Significant acceleration is achieved using this method without compromising on the accuracy. The CPFE twin models, stabilized and accelerated with the proposed numerical methods, predict the microstructure-property relations and extend the study of failure initiation in single and polycrystalline Mg alloys.